1. Field of the invention:
The present invention relates to a black photoconductive toner which has a good electrification property and photoconductivity, and exhibits excellent sensitivity to light in the wavelength range of semiconductor lasers.
2. Description of the prior art:
A copy image forming method using photoconductive toner has been attracting attention in recent years as a method of forming a copy image without using a photoconductor drum. The photoconductive toner is a toner prepared by dispersing or dissolving a photoconductive material, a sensitizer, etc. in a resinous binder and granulating such a mixture using a grinding or spray-dry technique. The toner is by itself provided with photoconductivity.
The following describes one example of an image forming method using such a photoconductive toner. In this method, the following steps (1) to (4) are sequentially performed:
(1) Charged photoconductive toner is made to adhere uniformly onto a conductive base, thereby forming a toner layer on the surface thereof. (2) An image exposure is performed. (3) A copy paper is superposed on the toner layer, and corona discharge is applied from the back of the copy paper to transfer the toner image onto the copy paper. (4) The transferred image is fixed to the copy paper.
In the above process, since the photosensitivity of the photoconductive toner is generally low, only a small difference is created between the electric charge of the exposed portion and that of the unexposed portion of the latent image formed through the exposure. To increase the difference in the electric charges, a method has been tried to apply corona discharge over the toner layer for enhancement of the surface potential thereof. This method, however, has not necessarily been successful in resolving the above shortcoming. Raising the proportion of the photoconductive material in the toner to enhance the photoconductivity of the toner has also been considered. This has in turn caused a deterioration of the electrification property of the toner, and as a result, led to a drop in the photosensitivity of the toner. It is therefore desired to develop a photoconductive toner which has a good electrification property and photoconductivity, and exhibits excellent photosensitivity.
The image forming system using the photoconductive toner has recently come to be considered for application to laser printers, in addition to the conventional application to analog image forming apparatus, because of the advantage that the entire apparatus can be designed to be extremely compact. Previously, gas lasers were used as the laser light source for the exposure system of laser printers. In recent years, however, semiconductor lasers are extensively used as the laser light source, since they have many advantages such as high and stable laser light intensity, directly controllable laser light intensity, low cost, etc. As the semiconductor laser produces a laser light having wavelengths in the range of 780 to 850 nm, i.e., in the near infrared to the infrared regions, researches are being carried out into photoconductive materials which have good sensitivity to light with the wavelengths in the range of 780 to 850 nm, pigments which are used as sensitizers to shift the photosensitivity range of the toner to the region of infrared light, and so on.
It has previously been proposed to shift the photosensitivity range of the toner to the region of infrared light by using zinc oxide as the photoconductive material and a cyanine pigment as the sensitizer. In this case however, the toner containing zinc oxide, a cyanine pigment, and a resinous binder gives a bluish green color, not black, therefore, it may be necessary to add a black pigment as a colorant. If carbon black, which is generally used as a black pigment, is added to the toner, the photosensitivity of the toner will drop, since carbon black absorbs light having wavelengths in the range of 780 to 850 nm, which is the wavelength range of the semiconductor lasers.
A method has also been considered to prepare a black toner using a developer and a heat sensitive black pigment which does not absorb light having the wavelength in the vicinity of 780 nm, but it has been found that this method causes a marked drop in the electrification property of the toner. Since the heat sensitive black pigment generally has lactone rings, it adheres to the surface of zinc oxide when the rings open because of coloring action, thus hampering the adhesion of the cyanine pigment.